1. Field of the Invention
This invention concerns a light irradiation device that uses a short-arc lamp to form a linear, long thin light irradiation region and an inkjet printer. In particular, the invention relates to a light irradiation device that forms a linear light irradiation region having a uniform irradiance on the article to be irradiated, and an inkjet printer, in which this light irradiation device is mounted, that records images or circuits and other patterns on a substrate by ejecting a light-curable liquid material onto the substrate.
2. Description of Related Art
Because it is possible to produce images more conveniently and cheaply than the gravure method, in recent years, the inkjet printing method has been adopted in a variety of printing fields including specialty printing, such as photographs, printing of various kinds, marking, and color filters.
In the inkjet printing method, it is possible to obtain high graphic quality by combining inkjet printers of the inkjet printing method that eject and control fine dots, inks with improved color reproduction, durability, and ejection properties, and specialty papers with greatly improved ink absorption, color development properties, and surface gloss.
These inkjet printers can be classified by the type of ink, but among them there is a light-cure inkjet method that uses light-curable inks that are cured by irradiation with ultraviolet and other light. The light-cure inkjet method is a relatively low-odor process and is noted for quick drying even with non-specialty papers and the ability to print even on recording media that do not absorb ink.
With inkjet printers of this light-cure inkjet type (called “inkjet printers” hereafter), a light source that irradiates light is mounted on the carriage along with the inkjet head that ejects ink in the form of small droplets onto the substrate; the carriage is moved with the light source lighting the substrate, and the ink is cured by irradiation with the light immediately after it impacts the substrate (see, for example, JP Pre-grant Patent Report Nos. 2005-246955 (corresponding to U.S. Patent Publication No. 2005/0168509), 2005-103852, & 2005-305742 and the article “Trends of UV Inkjet Printing,” Noguchi Hiromichi, Orikasa Teruo, Bulletin of the Japanese Society of Printing Science and Technology, Vol. 40, No. 3, p. 32 (2003).
Now, there have been attempts in recent years to use inkjet printers not only for printing of images as mentioned above, but also for forming electronic circuit patterns. In this case, the liquid material that is from the inkjet head is a material for making circuit boards, such as a light-curable resist ink; the substrate on which printing (that is, pattern formation) is performed is, for example, a printed-circuit board.
Formation of circuit patterns by means of resist ink, like record printing of images, has used a dry and cure reaction by means of UV or other light and the material ejected from the inkjet head is different from resist or ink, but the configuration of the inkjet printer equipment is the same.
Equipment that records images on a substrate using light-curable ink is explained below as an example of an inkjet printer.
FIG. 10(a) is a perspective view showing the schematic structure of the head portion of an inkjet printer; and FIG. 10(b) is a cross section, cut along the vertical plane of the light beam of the lamp of the light irradiation device 6 or 7 shown in FIG. 10(a). Now, FIG. 10(a) is drawn so that the interior of the light irradiation device is visible in order to facilitate the explanation that follows.
An inkjet printer 1 has a bar-shaped guide rail 2, and a carriage 3 is supported on this guide rail 2. The carriage 3 is moved by a carriage drive mechanism (not shown) back and forth along the guide rail 2 above a substrate 5. This direction is called the X direction below.
On the carriage 3 is mounted an inkjet head 4 to which are attached nozzles (not shown) that eject ink of various colors for color printing. Light irradiation devices 6, 7 are attached on both sides, in the direction of movement on the carriage 3, of the inkjet head 4, and the light irradiation devices 6, 7 irradiate the ink, which is the liquid material ejected onto the substrate 5 from the nozzles of the inkjet head 4, with ultraviolet light.
Now, the portion that comprises the inkjet head 4 and the irradiation devices 6, 7 is called the head portion 1a below.
In FIG. 10(a), when printing is being performed on the substrate while the carriage 3 is moving forward in the X direction shown in that figure, the ink from the inkjet head 4 of the head portion 1a is cured by means of the light from the light irradiation device 6. Further, when printing is being performed on the substrate while the carriage 3 is moving backward in the X direction, the ink from the inkjet head 4 of the head portion 1a is cured by means of the light from the light irradiation device 7.
The light irradiation devices 6, 7 have box-shaped covers 8 in which there are openings 20 facing the substrate 5, and within the covers there are long-arc type discharge lamps 90 which are linear light sources that run in the direction perpendicular to the X direction of movement of the carriage 3 (hereafter, the Y direction). The length of the light emitting tubes of the lamps 90 is roughly equal to the length in the Y direction of the inkjet head 4.
High-pressure mercury lamps or metal halide, for example, are used as these long-arc type discharge lamps.
There is a tubular reflector 110 that reflects the light (ultraviolet light) emitted from the lamp 90 on the side of the lamp 90 opposite the opening 20. The cross section of the reflector 110 has an elliptical shape as shown in FIG. 10(b); the discharge lamp 90 is located at the first focal point of the ellipse and the light (ultraviolet light) emitted from the lamp 90 is focused in linear form at the second focal point of the reflector 110, but irradiation by direct-projection light from the lamp 90 is added to that.
The substrate 5 is located so that it passes through the second focal point position of the reflector 110 or its vicinity, and the ink that impacts the substrate 5 is irradiated by light focused by the reflector 110.
Recently there has come to be a desire that the light (ultraviolet light) that cures the ink have even stronger irradiance in inkjet printers as described above.
The ink must have low viscosity, to some extent, to be ejected smoothly from the nozzles of the inkjet head. For that reason, if the ink is not cured (photopolymerized) immediately after impacting the substrate, the shape of the dot of ink will change after impact and image quality is reduced. In order to conduct curing (photopolymerization) quickly, it is desirable to irradiate light with a high peak irradiance so the polymerization reaction goes forward at once.
To meet this demand, consideration has been given to making the polymerization reaction go forward quickly by means of increasing the peak irradiance of the irradiating light from the light irradiation device. For example, JP Pre-grant Patent Publication No. 2005-246955 and corresponding U.S. Patent Publication No. 2005/0168509, cited above, disclose that it is possible to lessen the degree that the speed of ink curing drops because of oxygen, or in other words, it is possible to prevent a decrease in image quality by speeding up the ink curing process; it also states that it is possible to form a light irradiation region of equal size to that produced by a long-arc type discharge lamp and that a microwave UV lamp is effective in yielding higher irradiance than a long-arc type discharge lamp. The peak irradiance of the microwave UV lamp mentioned in JP Pre-grant Patent Publication No. 2005-246955 and corresponding U.S. Patent Publication No. 2005/0168509 is in the range of 1000 to 1200 mW/cm2.
Further, the JP Pre-grant Patent Publication 2005-103852, cited above, describes technology to locate lenses between multiple light source lamps, located on a plane, and the substrate, and to increase the peak irradiance irradiating the substrate by means of focusing light from the light source lamps to irradiate the substrate.
However, even when irradiating with light focused from light source lamps using optical elements such as lenses and mirrors, the peak irradiance yielded will be limited unless the radiance of the light source lamps themselves is increased; this is the case even when using the microwave UV lamps as described in JP Pre-grant Patent Publication No. 2005-246955 and corresponding U.S. Patent Publication No. 2005/0168509.
It is thought that there will be further demands to increase the peak irradiance of the light irradiating the substrate in the future; to satisfy these demands it will be necessary to further increase lamp radiance. However, the reality is that it is technically difficult to further increase the radiance of long-arc lamps, which have large light-emitting tubes, or microwave UV lamps.
Further, there are also the following problems in the inkjet printers described above. That is, in a conventional inkjet printer having the configuration shown in FIG. 10(a), for example, the discharge lamps 90 face the substrate 5 directly, through the openings 20.
Accordingly, the light from the discharge lamps 90 irradiates the substrate 5 directly, but the light emitted from the discharge lamps 90 includes light from the visible and infrared regions that is not needed for curing ultraviolet-cured inks, and thermal radiation from the arc tube of the discharge lamps 90, which reach high temperatures when the lamps are lit, is also incident on the substrate 5, so that the substrate 5 is heated by means of the light from the visible and infrared regions and the thermal radiation from the lamp seal portions.
Materials that are easily deformed by heat—paper, resin, or film, for example—are often used as the substrate 5, so if one simply uses lamps with higher power to increase the radiance, the effect on the substrate 5 of heat from light in the visible and infrared regions and from thermal radiation will be even greater, the temperature of the substrate will raise even higher, and this will be the source of degraded printing quality as such results in deformation.
One possibility for dealing with such problems is to place a reflecting mirror (also called a cold mirror), on which is formed a vapor deposition layer that reflects only light of the wavelengths for curing the ink and allows other wavelengths to pass through, between the discharge lamp and the substrate; because only the light reflected by this reflecting mirror irradiates the substrate, the effect of heat on the substrate is reduced.
However, putting such a reflecting mirror in place lengthens the optical path from the discharge lamp to the substrate by that much more; in the case of a long-arc type discharge lamp, for example, that makes it impossible to focus light in the lengthwise direction of the discharge lamp, and so the area irradiated by the light (the light irradiation region) is expanded, reducing the efficiency of light use and also making it impossible to obtain high enough irradiance in the light irradiation surface (the substrate surface).
As stated above, the reality is that it is difficult to increase the peak irradiance in the light irradiation surface beyond the conventional level and devise improvement of the ink curing process in inkjet printers that use the light-cure inkjet method.
In order to solve such problems, it is proposed in commonly owned, co-pending U.S. patent application Ser. No. 11/738,160, to use a light irradiation device having, as the light source lamp, a short-arc type discharge lamp that has greater radiance than long-arc type discharge lamps, and focusing the light from the lamps to extend the light in a line. FIG. 11 shows an example of the configuration of the light irradiation device proposed in that co-pending U.S. patent application.
The light from a short-arc type discharge lamp 9 is first reflected by a reflector 111 that has a reflective surface that is a paraboloid of revolution placed to surround the lamp 9. Next, the light that has been reflected by the reflector 111 is reflected by a mirror 112 that has a cylindrical reflecting surface of which a cross section is parabolic.
The light from the lamp 9 is reflected as collimated light by the reflector 111 that has a reflecting surface that is a paraboloid of revolution. When the collimated light is reflected by the mirror 112 that has a reflecting surface of which a cross section is parabolic, the light is focused on the light irradiation surface 13 in a linear direction perpendicular to the plane of the paper in FIG. 11.
In the light irradiation device described above, however, no particular consideration was given to the uniformity of irradiance in the lengthwise direction of the linearly focused light. For that reason, it is thought, the irradiance distribution in the light irradiation region has high irradiance in the center and declining irradiance towards the edges. In order to process uniformly across the full irradiation region, it is necessary to form a light irradiation region with good uniformity of irradiance. Poor uniformity of irradiance results in the problem that uniform processing across the full irradiation region is not possible.